A CT scan of the heart creates detailed cross-sectional images that can reveal calcium buildup in your coronary arteries, plaque that hasn’t yet calcified, narrowing or blockages in blood vessels, and structural problems with the heart’s chambers, valves, and surrounding tissue. There are two main types of cardiac CT, and each one shows different things.
Two Types of Heart CT
Most people getting a heart CT will have one of two scans: a coronary calcium score or a coronary CT angiography (CCTA). They use the same basic technology but answer different questions.
A coronary calcium score is a quick, no-contrast scan that measures calcified plaque in your coronary arteries. It gives you a single number, called an Agatston score, that reflects how much calcium has accumulated. A score of zero means no detectable calcium and very low risk. Scores of 1 to 99 indicate mildly increased risk, 100 to 299 moderately increased risk, and 300 or above moderate to severe risk. Because this scan only detects calcified plaque, it can underestimate total plaque burden. Younger patients in particular may have non-calcified plaque that this scan misses entirely.
A CCTA goes further. It uses an iodine-based contrast dye injected into a vein to light up the inside of your coronary arteries in real time. This lets it detect both calcified and non-calcified plaque, measure how much an artery has narrowed, and assess overall plaque volume and composition. It’s the only noninvasive test that can characterize both types of plaque, which makes it especially useful for evaluating chest pain or unclear symptoms.
What CCTA Reveals About Plaque and Blockages
When contrast dye fills your coronary arteries, the scan can distinguish between different types of plaque based on how dense they appear on the image. Plaque is categorized by its density, measured in Hounsfield units. Very low-density plaque is prognostically important because it’s associated with a higher risk of future cardiac events. Denser, calcified plaque is easier to measure reliably but may be less useful for tracking how well treatment is working.
Beyond identifying plaque type, CCTA can measure plaque burden (the proportion of plaque relative to overall vessel size) and total plaque volume (the difference between the outer vessel wall and the open channel where blood flows). These measurements help determine whether a narrowing is mild or severe enough to restrict blood flow. Compared to invasive catheter-based angiography, CCTA has a diagnostic sensitivity of about 94% and specificity of 97% for detecting significant coronary artery disease. Its negative predictive value is 99%, meaning that if the scan looks clear, it almost certainly is.
Heart Valves, Chambers, and the Aorta
A heart CT doesn’t stop at the coronary arteries. It captures the entire heart and surrounding structures in each image set, which means radiologists routinely evaluate several other findings.
- Heart valves: The scan can show thickened or calcified valve leaflets, restricted valve opening, and structural variants like a bicuspid aortic valve (which has two cusps instead of the normal three). For patients with prosthetic valves, it can detect complications like blood clots, infection-related damage, or valve separation from surrounding tissue.
- Heart chambers: Using images captured at multiple points in the heartbeat cycle, the scan can assess chamber size and overall pumping function.
- Aorta: The scan reliably identifies aortic aneurysms (the ascending aorta is considered dilated above 4 cm), aortic dissection (a tear creating a flap inside the vessel wall), and focal narrowing of the aorta.
- Pericardium: CT detects fluid around the heart with greater sensitivity than ultrasound, particularly when the fluid is localized to one area. It also identifies pericardial thickening (above 4 mm suggests inflammation) and calcification from chronic pericarditis.
Congenital Heart Abnormalities
Heart CT is particularly good at mapping coronary artery anomalies, where arteries take unusual paths from the aorta. When the scan is synchronized to your heartbeat using an ECG signal, it can trace the exact course of each coronary artery and its relationship to surrounding structures. Even without ECG synchronization, studies have found that the origins and proximal segments of the coronary arteries are visible in about 82% of patients with congenital heart disease.
For more complex congenital conditions, the scan allows radiologists to systematically evaluate the heart’s chambers, the connections between chambers and great arteries, septal defects, and abnormalities of the pulmonary veins or aortic arch.
How to Prepare for a Heart CT
Preparation depends on which scan you’re having. A calcium score requires almost no prep. A CCTA typically requires fasting for at least 8 hours beforehand, though water is fine. You’ll need to avoid caffeine for 12 hours before the scan because it raises your heart rate and blurs the images. Your care team may give you a beta blocker before the scan to slow your heart rate further, since a slower, steadier rhythm produces sharper images.
For CCTA, an IV line delivers iodine contrast dye during the scan. If you have kidney disease, the contrast poses a real risk. About 8% of people with mildly reduced kidney function develop contrast-related kidney injury, and that figure rises to 27% in people with severely reduced function. Your doctor should check your kidney function beforehand. People taking metformin for diabetes are typically told to stop it on the day of the scan and for 48 hours afterward. If you’ve had allergic reactions to iodine contrast in the past, that’s important to mention before scheduling.
Radiation Exposure
All cardiac CT scans involve radiation, but the dose varies significantly by scan type. A coronary calcium score delivers roughly 3 millisieverts (mSv), which is about 150 times the dose of a single chest X-ray but still relatively modest. A coronary CT angiogram delivers around 16 mSv, putting it at the higher end of diagnostic imaging.
For context, the FDA estimates that a CT scan delivering 10 mSv may increase the chance of fatal cancer by about 1 in 2,000. That’s a small addition to the baseline risk of fatal cancer in the U.S. population, which is roughly 1 in 5. For most people being evaluated for heart disease, the diagnostic value of the scan far outweighs this small incremental risk.